(1) Field of the Invention
The present invention relates to a control apparatus and control method for an optical switch with a three-dimensional structure using micro mirrors (referred to hereunder as MEMS mirrors) manufactured by applying micro machining (MEMS: Micro Electro Mechanical System) technology. In particular, it relates to a controlling technique for compensating for a change in characteristic due to a temperature variation.
(2) Description of Related Art
In recent years, the Internet rapidly expanded has required the implementation of flexible and highly reliable optical networks for rapidly increased communication traffic. Consequently, various optical networks have been proposed provided that the routing is performed per wavelength units. For example, there are optical networks that perform the adding and dropping, the cross connection and the like, of optical signals per wavelength unit.
However, in the optical network as described above, since the number of connected nodes and the number of wavelengths are increased, it is considered that very large scaled optical switches are required for switching optical signals. From this standpoint, an attention has been given to an optical switch with a three-dimensional structure using MEMS mirrors, as an optical switch suitable for large scaling, from the standpoint of coupling losses and the like.
As one controlling technique for MEMS mirrors in the above described three-dimensional type optical switch, the present applicant has proposed a system in which an optimum driving condition of a MEMS mirror is stored in advance in a memory as an initial value corresponding to an optical path connected by a three-dimensional type optical switch, and after receiving a command to connect or switch the optical path, the initial value is read out by accessing the memory, a drive signal corresponding to the initial value is given to the MEMS mirror, and furthermore, a drift due to a temperature variation and the like is compensated for by a feedback control, to maintain an optimum driving condition (refer to Japanese Unexamined Patent Publication No. 2002-236264, Japanese Patent Application No. 2001-216297, Japanese Patent Application No. 2002-242290). According to these prior applications, a three-dimensional type optical switch can be controlled with low optical loss, high accuracy and high speed, thus enabling a small sized, large capacity optical exchange, or the like, to be developed.
Incidentally, the conventional controlling technique as described above still has the following problems. That is, in the case of an optical switch with a three-dimensional structure, since the accuracy in angle of inclination required for a MEMS mirror is strict, if it is shifted from an optimum angle of inclination by even a little, an optical coupling loss is increased significantly. Accordingly, there is a possibility that the optical coupling loss is changed significantly due to a slight variation caused by the temperature variation in mechanical or electrical characteristic of the MEMS mirror, or a slight variation which is caused by the temperature variation of a spatial optical coupling system due to being a three-dimensional structure.
On the other hand, in actual optical networks, there is a requirement for protection times of 50 ms or less over the whole network, for example, so it is necessary to realize the switching of optical paths in several ms in individual optical switch unit. However, if the initial value of the driving condition given to the MEMS mirror when switching optical paths is deviated significantly from the optimum value due to the above-described temperature variation, an amount of compensation in the feedback control which is performed after initialization is increased. Therefore, there is a problem in that the high speed switching of optical paths is difficult.